Question: Simplify the following expression: $z = \dfrac{9a}{7} - \dfrac{7a}{2}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $7$ and $2$ $\lcm(7, 2) = 14$ $ z = \dfrac{2}{2} \cdot \dfrac{9a}{7} - \dfrac{7}{7} \cdot \dfrac{7a}{2} $ $z = \dfrac{18a}{14} - \dfrac{49a}{14}$ $z = \dfrac{18a -49a}{14}$ $z = \dfrac{-31a}{14}$